In order to lock two interfitting parts together, it is standard to form them with bores that align when they are properly fitted together and insert a ball-type quick-release lock pin into the aligned bores. This pin is normally only stressed in shear and therefore can resist substantial forces.
Such a pin typically comprises an at least partially tubular shaft centered on and extending along an axis and formed at a front end with at least one radially outwardly open aperture. A locking element, typically a ball, is captured in this aperture, normally by providing a lip along its outer edge, so that it can move between a rear locking position projecting from the aperture past an outer surface of the pin and an inner position wholly withdrawn in the aperture and not projecting at all. When in the rear locking position, the element typically fits in an undercut of the bore of one of the parts it is locking together so that it cannot be withdrawn.
While the ball in some systems is spring loaded, for most quick-release structures the pin is provided with an axially extending core rod itself formed with an outwardly open recess. The rod is shiftable axially between an actuated position with its recess aligned with the lock element and into which the element can fit in the inner position and an unactuated position with its outer surface bearing on the ball and forcing the ball into the rear locking position. A spring is braced against the ball and/or the rod to urge the rod into the unactuated position and the ball into the rear locking position. Normally the actuating rod projects from a rear end of the tubular pin so that it can be operated by the user. In the simplest system a ring is fitted to a projecting rear end of the actuating rod so that a pull on this ring releases the lock element and lets the ball retract so the pin can be pulled out. GB 971,537 describes a system where the rear end of the pin forms a chamber around a piston carried on the rear end of the actuating rod so the lock pin can be actuated hydraulically or pneumatically.
The problem with these lock pins is that they are frequently fairly complex and expensive to manufacture. Furthermore the releasing action is often uniaxial, for example comprises a simple pull on the ring of the actuating rod, so that accidental release is a problem.